Retromer is a conserved multi-protein complex that is required for retrograde transport of transmembrane proteins. It was initially characterized in yeast as a pentameric complex required for the recycling of the transmembrane receptor VPS10 to the trans-Golgi, and was subsequently shown to be conserved in flies, worms and humans. In humans, retromer consists of a cargo-recognition subcomplex made up of VPS35, VPS26 and VPS29 and a membrane-targeting subcomplex containing a heterodimer of SNX proteins (SNX1 or 2 paired with SNX5 or 6). The SNX proteins contain a BAR domain that is believed to promote membrane curvature, and SNX-BAR proteins are thought to aid in the formation of endosomal membrane tubules into which cargo is loaded (reviewed in Pfeffer, 2001; Seaman, 2012).
Retromer is required for the recycling of WLS to the Golgi to allow further rounds of WNT-ligand delivery to the plasma membrane (Coudreuse et al 2006; Belenkaya et al 2008; Port et al, 2008). In the absence of essential retromer component VPS35 or VPS26, WLS is diverted to the MVB and degraded, and WNT ligand accumulates inside the cell; overexpression of WLS is sufficient to rescue the vps35 defect in WNT signaling (Belenkaya et al, 2006; Franch-Marro et al, 2008). WLS and retromer colocalize on endosomal structures and WLS and VPS35 co-precipitate in pull down studies (Belenkaya et al, 2006; Port et al, 2008; Franch-Marro et al, 2008).
Several recent studies have suggested that WLS recycling depends on a WNT-specific retromer in which the SNX-BAR proteins of the classic complex are replaced by SNX3 (Zhang et al, 2011; Harterink et al, 2011; reviewed in Johannes and Wunder, 2011). Unlike SNX1/2/5/6, SNX3 does not contain a BAR domain, and WLS is suggested to accumulate in endocytic vesicles rather than in the tubular structures of the 'classic' retromer (Harterink et al, 2011; Zhang et al, 2011). SNX3 is recruited from the cytosol to the early endosome through the interaction of its PX domain with PIP3 in the membrane. Mutation of critical residues in the PX domain abolish the interaction with PIP3 and ablate endsomal recruitment of SNX3 (Xu et al, 2001; Zhang at al, 2011; Harterink et al, 2011). SNX3 has been shown to co-immunoprecipitate with VPS35 and VPS26, and some studies have also shown a direct interaction between SNX3 and WLS (Zhang et al, 2011; Harterink et al, 2011).
